Add GDPR declaration for pyrefly.detection telemetry event (microsoft#26053)

## Summary

Adds a `__GDPR__` declaration for the Pylance `pyrefly.detection`
telemetry event so it is allow-listed and reaches the telemetry
pipeline.

## Background

Pylance emits a `PYREFLY.DETECTION` event (sent through the
`ms-python.python` reporter as `ms-python.python/pyrefly.detection`)
during background Pyrefly detection, to measure the addressable audience
that could be offered a switch to Pyrefly. The event shipped in Pylance
`2026.2.109` (pre-release) and `2026.3.1` (stable) and fires
unconditionally on the **same** reporter as `language_server.ready`.

Despite millions of users on those builds emitting the sibling
`language_server.ready` event, `pyrefly.detection` showed **zero** rows
in telemetry. Root cause: the new event name was not declared in this
GDPR allow-list, so it was scrubbed before ingestion.
`language_server.ready` is declared here and flows fine;
`pyrefly.detection` was missing.

## Change

Declares `pyrefly.detection` with its properties:

- `available`, `haspyreflytoml`, `haspyprojectsection`,
`hasenvironmentbinary`, `promptdismissed`, `notificationenabled` — the
event-specific fields emitted by Pylance's `_reportDetectionTelemetry`.
- `lsversion`, `failed` — common properties, mirroring the sibling
`language_server.ready` declaration.

All properties are `SystemMetaData` / non-personal (booleans and the LS
version).

## Impact

No runtime code change. Once a Python extension build containing this
declaration reaches users, data will begin flowing from already-deployed
Pylance clients that are already emitting the event.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: rchiodo

src/client/telemetry/pylance.ts

@@ -490,3 +490,20 @@
    "failed" : { "classification": "SystemMetaData", "purpose": "PerformanceAndHealth" }
 }
 */
+/**
+ * Telemetry event sent during background Pyrefly detection to measure the audience that could be
+ * offered a switch to Pyrefly. Reported once per workspace folder (per availability) for users who
+ * are not already on a Pyrefly diagnostics source.
+ */
+/* __GDPR__
+   "pyrefly.detection" : {
+      "available" : { "classification": "SystemMetaData", "purpose": "FeatureInsight", "owner": "rchiodo" },
+      "haspyreflytoml" : { "classification": "SystemMetaData", "purpose": "FeatureInsight", "owner": "rchiodo" },
+      "haspyprojectsection" : { "classification": "SystemMetaData", "purpose": "FeatureInsight", "owner": "rchiodo" },
+      "hasenvironmentbinary" : { "classification": "SystemMetaData", "purpose": "FeatureInsight", "owner": "rchiodo" },
+      "promptdismissed" : { "classification": "SystemMetaData", "purpose": "FeatureInsight", "owner": "rchiodo" },
+      "notificationenabled" : { "classification": "SystemMetaData", "purpose": "FeatureInsight", "owner": "rchiodo" },
+      "lsversion" : { "classification": "SystemMetaData", "purpose": "FeatureInsight", "owner": "rchiodo" },
+      "failed" : { "classification": "SystemMetaData", "purpose": "PerformanceAndHealth", "owner": "rchiodo" }
+   }
+*/